BIOS is the software that starts your computer before Windows or any other operating system loads

BIOS stands for Basic Input/Output System. It is a small program stored on a chip inside your computer that runs the moment you press the power button — before your operating system even exists on the screen. Think of it as your computer's startup instructions. BIOS checks that all your hardware (your hard drive, RAM, keyboard, monitor) is connected and working, then it finds your operating system and hands control over to it.

Without BIOS, your computer would not know how to talk to its own parts. It would be like a car with an engine but no ignition system — the parts are there, but nothing tells them to work together. BIOS is what makes that handshake happen.

Key Takeaways

  • BIOS runs before your operating system loads and checks that your hardware is working.
  • You can enter BIOS setup by pressing a specific key (usually Delete, F2, or F12) during startup, before Windows appears.
  • Most people never need to change BIOS settings, but you might adjust boot order if you want to start from a USB drive or external disk.
  • BIOS settings are stored on a small battery-powered chip, so they survive even when your computer is unplugged.
  • Modern computers increasingly use UEFI instead of traditional BIOS, though UEFI does the same job with a different interface.

How BIOS starts your computer

When you power on your computer, the CPU (processor) automatically runs the BIOS code. BIOS then performs a test called POST — Power-On Self-Test — which checks that your RAM, hard drive, keyboard, and other core parts are present and responding. If something is missing or broken, BIOS will beep or display an error message and stop.

Once POST passes, BIOS looks for your operating system. It checks the devices you have told it to check in a specific order — usually your hard drive first, then a USB drive, then a CD drive. When BIOS finds a bootable drive (one that contains an operating system), it loads that system into memory and steps out of the way. From that moment on, Windows, macOS, or Linux is in control, and BIOS stays quiet until the next time you restart.

Entering BIOS setup and what you see there

You can enter BIOS setup by restarting your computer and pressing a key during the first few seconds of startup. The key varies by manufacturer — common ones are Delete, F2, F10, or F12. Your computer usually displays a message like "Press Delete to enter Setup" during the boot screen, so you will know which key to use.

Inside BIOS setup, you will see menus with options like boot order, date and time, hard drive detection, and security settings. The layout and options differ between manufacturers (Dell, HP, Lenovo, Asus, etc.), but the basic structure is similar. Most BIOS interfaces use arrow keys and Enter to navigate, not a mouse. If you have never been in BIOS before, the screens can look intimidating, but you do not need to change anything unless you have a specific reason.

Common reasons to change BIOS settings

The most frequent reason people enter BIOS is to change the boot order — the sequence in which your computer checks for an operating system. If you want to install Windows from a USB drive or run a repair tool from an external disk, you need to tell BIOS to check the USB port before the hard drive. You would move "USB Drive" to the top of the boot order list, save, and restart.

Other reasons include enabling or disabling hardware (like turning off a built-in camera or microphone for security), updating the BIOS itself to fix bugs or add features, or adjusting power settings for a laptop. Some people also enter BIOS to check that their hard drive or RAM is being recognized correctly. In most cases, though, the default BIOS settings work fine and do not need to be touched.

BIOS versus UEFI — what changed

Newer computers use UEFI (Unified Extensible Firmware Interface) instead of traditional BIOS. UEFI does the same job — it starts your computer and loads your operating system — but it is faster, supports larger hard drives, and has a more modern interface that sometimes includes a mouse and graphics. Many UEFI systems still call themselves "BIOS" in everyday language, so the terms are often used interchangeably.

From a user's perspective, the difference is minor. You still restart, press a key to enter setup, and adjust boot order or other settings the same way. The menus look different and may be easier to navigate, but the purpose is identical. If your computer was made in the last ten years, it almost certainly uses UEFI, though it may still display "BIOS Setup" on the screen.

What happens if BIOS settings get corrupted

BIOS settings are stored on a small battery-powered chip called CMOS. If that battery dies or the chip is damaged, your computer may forget its settings and revert to factory defaults. You might see error messages about the date being wrong or the hard drive not being detected. In most cases, you can straightforward re-enter BIOS setup and restore the correct settings.

In rare cases, the BIOS code itself becomes corrupted — usually from a failed update or a power loss during a restart. This is more serious and may require a technician to reflash the BIOS chip. However, this is uncommon on modern computers, which have safeguards against corruption. If your computer will not start at all and you suspect a BIOS problem, contact the manufacturer or a repair shop rather than trying to fix it yourself.

Why you should not change BIOS settings unless you know what you are doing

BIOS controls fundamental hardware behavior. Changing the wrong setting can prevent your computer from starting, disable a device you need, or cause instability. For example, disabling your hard drive in BIOS will make your operating system invisible, and your computer will not boot. Overclocking settings (which speed up your processor) can cause overheating and hardware failure.

The safest approach is to write down or take a photo of your current BIOS settings before you change anything, so you can restore them if something goes wrong. Most BIOS menus also have a "Load Defaults" or "Reset to Factory Settings" option that will undo your changes. If you are unsure whether a setting is safe to change, leave it alone or search for the specific setting name and your computer model online before touching it.

Frequently Asked Questions

How do I know if my computer uses BIOS or UEFI?

On Windows, open System Information (search for it in the Start menu) and look for "BIOS Mode" or "Firmware Type." It will say either "UEFI" or "Legacy" (which means traditional BIOS). On a Mac, you do not interact with BIOS or UEFI directly — Apple handles firmware differently. On Linux, you can check by running a terminal command, but most modern Linux systems work with UEFI automatically.

Can I update my BIOS?

Yes, manufacturers release BIOS updates to fix bugs, improve performance, or add support for new hardware. You can usually read updates from the manufacturer's support website by entering your computer model. Follow their instructions carefully, as a failed BIOS update can be serious. Most updates happen automatically on modern computers, so you may not need to do anything.

What if I accidentally changed a BIOS setting and my computer will not start?

Restart your computer and re-enter BIOS setup using the same key as before. Look for an option called "Load Defaults," "Reset to Factory Settings," or "Restore Defaults." Select it, save, and restart. This will undo your changes and return BIOS to its original state. If your computer will not let you enter BIOS at all, you may need to contact the manufacturer or a repair technician.

Does BIOS affect my computer's speed or performance?

In most cases, no. BIOS runs only during startup and then steps aside. However, some BIOS settings — like enabling or disabling hardware acceleration, or changing power profiles — can affect performance once your operating system is running. The default settings are optimized for most users, so changing them rarely improves speed and can sometimes slow things down.

Is BIOS a virus or malware?

No. BIOS is legitimate firmware made by your computer's manufacturer. However, malware can theoretically infect BIOS, which is why keeping your system updated and avoiding untrusted downloads is important. If you suspect a BIOS infection, run a full antivirus scan and contact a technician. This is extremely rare on modern computers with security features.